1. Field of the Invention
The present invention relates to a heat-sensitive recording material having an excellent dispersion quality for leuco dyes to be used for heat-sensitive color-developing layers and showing an excellent whiteness degree in ground color.
2. Description of the Related Art
Various recording materials have been studied, developed and put to practical use in the field of information recording in response to the diversification of information and expansion of needs in recent years. Particularly, heat-sensitive recording materials have advantages such as: (a) simple image recording is possible only by a heating process; and (b) required device mechanisms are simple and easily downsized, as well as recording materials are easily handled and are available at low cost. Thus, they have been used in a large number of different technological fields including information processing (outputs of desktop calculators, computers and so on), recorders for medical instruments, low and high-speed facsimile machines, automatic ticket vending machines (railway tickets, entry tickets, etc.), heat-sensitive copying machines and labels to be used in POS systems.
While thermal printers are normally employed to record various pieces of information on heat-sensitive recording materials, the operating speed of thermal printers has been improved in response to the increasing demand for heat-sensitive recording materials. Thermal printers have a thermal head in the inside and image information is formed on a heat-sensitive recording material as the thermal head is brought into contact with and apply thermal energy to the surface of the heat-sensitive recording material. Therefore, it is important that the heat-sensitive recording material shows a high coloring sensitivity in order to realize high-speed printing.
Various techniques have been proposed to improve the coloring sensitivity. According to one of such techniques, a leuco dye that is a coloring material is crushed into a small particle size so that it may be molten and color-developed with ease when heated by a thermal head.
A leuco dye is normally contained in a heat-sensitive color-developing layer of a heat-sensitive recording material. However, in order to uniformly disperse the leuco dye in the coating solution to form the heat-sensitive color-developing layer, the leuco dye that is an originally powdery solid has to be crushed into fine particles and put into a state of liquid dispersion by means of a dispersing mill. The coloring sensitivity of leuco dye can be improved by applying a large shearing force or prolonging the dispersing time in order to dimensionally minimize leuco dye particles in the process of dispersing the leuco dye.
There have been proposals of adding water soluble resin such as polyvinyl alcohol when dispersing leuco dyes for the purpose of raising the dispersion efficiency or stably holding leuco dye particles in a liquid.
For instance, Japanese Patent Application Laid-Open (JP-A) No. 59-159394 describes that a heat-sensitive recording material that is free from the problem of sticking and adhesion of sediments and operates well with thermal heads can be obtained by using a fluoran compound having a substituted or an unsubstituted amino group at least at 3- and 7-positions and carboxylic group modified polyvinyl alcohol as a dispersing agent for the fluoran compound.
Japanese Patent Application Laid-Open (JP-A) No. 03-173680 describes that a heat-sensitive recording material showing an excellent coloring sensitivity can be obtained by using polyvinyl alcohol and isobutylene-maleic anhydride ammonium as leuco dye a dispersing agent and dispersing the leuco dye down to a particle diameter of 0.7 μm.
Japanese Patent Application Laid-Open (JP-A) No. 2000-6520 describes that a heat-sensitive recording material showing an excellent coloring sensitivity and a high anti-moisture conservation property for colored images can be obtained by using carboxy-modified polyvinyl alcohol or sulfonic acid-modified polyvinyl alcohol along with an ionomer type resin as leuco dye a dispersing agent.
However, as the coloring sensitivity is improved, the texture whiteness degree falls remarkably as antinomy when dyes are micronized (down to an average particle diameter of 0.3 μm or less) by using any of the above-described techniques in an attempt to further improve the coloring sensitivity in order to meet the demand for a higher coloring sensitivity that has been increasing in recent years. The underlying mechanism of the antinomy is that, when a heat-sensitive solution (a mixture solution of a leuco dye dispersion and a developer dispersion) is prepared by using a highly micronized leuco dye, the contact area of the developer and the dispersion is increased to weaken the structure of the coloring material and reduce the texture whiteness degree of the heat-sensitive recording material.
As another known technique, Japanese Patent Application Laid-Open (JP-A) No. 2003-266950 describes that a heat-sensitive recording material that shows a high texture whiteness degree and provides an excellent stability for both the colored image and the texture can be obtained with little liquid fog (weak coloring due to contact of dye dispersion and developer dispersion) by dispersing a leuco dye by making it contain an anionic surfactant as a dispersing agent, reducing the average particle diameter of the leuco dye to between 0.10 μm and 0.30 μm and using polyvinyl alcohol, polyacrylsulfonic acid metal salt or partially saponified polyvinyl alcohol as polymer a dispersing agent. However, the above-described technique is accompanied by a number of drawbacks including that the texture whiteness degree is not sufficient, that bubbles in the leuco dye dispersion can hardly move out because the remarkably raised viscosity of the dispersion to consequently give rise to a defective application of the dispersion, that it can be difficult to feed the dispersion in the manufacturing process and that the stability of the dispersion falls with time (the particle size grows and the coloring sensitivity falls as time elapses).